1. Field of the Invention
The present invention relates to a screw rotor used in an oil-flooded screw compressor driven by a female rotor.
2. Description of the Related Art
Although a popular type of oil-flooded screw compressor is driven by a male rotor, an oil-flooded screw compressor driven by a female rotor to obtain a large number of revolutions is also well known. In the oil-flooded screw compressor, about 90% of input power is consumed by the male rotor and the remainder, about 10%, is consumed by the female rotor. Accordingly, in the oil-flooded screw compressor driven by the male rotor, 10% of input power is transmitted from the male rotor to the female rotor at a contact portion between tooth surfaces of the male and female rotors.
On the other hand, in the oil-flooded screw compressor driven by the female rotor, 90% of input power is transmitted from the female rotor to the male rotor at the contact portion. Therefore, much contact stress, what is called, Hertz stress acts on the contact portion, which causes pitting if the area of the contact portion is small. As is well known, when a convex tooth surface and a concave tooth surface are in contact with each other, the Hertz stress is proportional to the square root of the difference between the reciprocals of radii of curvature of the tooth surfaces. Therefore, the oil-flooded screw compressor driven by the female rotor is particularly required to minimize the Hertz stress at the contact portion, and it is important that the rotor tooth surfaces at the contact portion be equal in curvature. When the curvatures are equal, no Hertz stress arises, which makes it possible to prevent pitting.
Japanese Unexamined Patent Publication No. 60-153486 discloses a screw rotor in which tooth surfaces at a contact portion are equal in curvature. In this screw rotor, the contact portion is shaped like an arc whose center is located on a pitch circle. FIG. 4 shows this screw rotor. A tooth surface enclosed by a circle X is shaped like an arc having the center at a point O on the pitch circle. FIGS. 5 and 6 are enlarged views of the circle X shown in FIG. 4. In FIGS. 4, 5 and 6, M denotes a male rotor, F denotes a female rotor, and P.sub.M and P.sub.F denote pitch circles of the male rotor M and the female rotor F, respectively.
In this screw rotor, if the center distance between the male and female rotors M and F has no error as designed, the male and female rotors M and F are in uniform planar contact with each other over a wide range as shown in FIG. 5. It is actually impossible, however, to reduce the error to zero. If the center distance is not equal to the designed value, the rotors at the contact portion are in local contact, which is shown as point contact in radial section, as shown by the arrow Y in FIG. 6.
Therefore, in the case of the screw compressor driven by the female rotor, pitting of the screw rotor is inevitable in actuality.